Method and apparatus for shutting in offshore wells

ABSTRACT

An offshore exploratory well is plugged, cut off just above the mud line, and left until equipment for completion of the well is ready. A cementing sub installed just above the mud line in one or more casing strings has a sleeve surrounding a joint in the casing strings and isolating that joint from fluids in the surrounding annulus between casing strings. Before leaving the well, the casing strings are cut off through the cementing subs above the joint within the subs to leave a portion of casing connected in the joint. That portion of casing can be removed to leave threads for tying back into the well.

United States Patent Stone 1451 May 30,1972

[54] METHOD AND APPARATUS FOR SHU'ITING IN OFFSHORE WELLS 1,880,269 10/1932 ...28 5/45X 2,301,495 11/1942 Abegg 85/286X I 1111|Il 1111 2,126,075 8/1938 Wright ..175/325 3,017,934 1/1962 Rhodes et a]. ..175 7 x 3,330,339 7/1967 Johnson... 166/.6 3,421,580 l/l969 Fowler ..166/.5

Pn'mary Examiner-Marvin A. Champion Assistant ExaminerRichard E. Favreau Attorney-Meyer Neishloss, Deane E. Keith and Paul L. Tillson [ 5 7] ABSTRACT An offshore exploratory well is plugged, cut off just above the mud line, and left until equipment for completion of the well is ready. A cementing sub installed just above the mud line in one or more casing strings has a sleeve surrounding a joint in the casing strings and isolating that joint from fluids in the surrounding annulus between casing strings. Before leaving the well, the casing strings are cut off through the cementing subs above the joint within the subs to leave a portion of casing connected in the joint. That portion of casing can be removed to leave threads for tying back into the well.

8 Claims, 4 Drawing Figures Patented May 30, 1972 2 Sheets-Sheet 2 //v1/E/vmQ. wee/1. 0. 570M? METHOD AND APPARATUS FOR SHU'ITING IN OFFSHORE WELLS This invention relates to offshore wells for the production of oil or gas, and more particularly to a method and apparatus for shutting in the wells in a manner that allows tying back into the wells.

The daily rental charge for mobile drilling rigs capable of drilling offshore in water of substantial depth, for example, 100 to 150 feet, may be as much as $10,000 a day. For this reason, it is essential to avoid holding such a rig on a well while waiting for equipment for completion of the well. When mobile drilling rigs are used in exploratory drilling, frequently the mobile rig is held on the well only long enough to determine whether or not the well is productive. If the well will be commercially productive, it is ordinarily cut off a short distance above the mud line to eliminate a hazard to shipping and the mobile drilling rig moved off the well. When production platforms and equipment are ready and are installed above the well, the completion of the well can be accomplished by small rigs for which the rental is only a small fraction of the rental charge for the mobile rigs.

Equipment has been developed for suspending casing in the well at or shortly above the mud line in a manner that will allow later tie-in to the well. The equipment heretofore available has been expensive. Moreover, the annulus between the surface casing and the next smaller string of casing and between that latter casing and the next smaller string of casing is not maintained full of cement during subsequent drilling into deep formations which may be exposed to high pressure. The manipulation of the casing that has been required to operate the equipment heretofore available may result inleaks which could permit the high pressure of deep formations to reach the larger casing. The larger casing is not designed to handle such high pressures; hence, there is danger of rupturing the surface casing or the next smaller string of easing.

This invention resides in a method and apparatus for cutting off an offshorewell at a short distance above the mud line in which the annuli between the successive strings of casing are filled to the wellhead with cement throughout the drilling operation. At least one string of casing that is to be extended to the wellhead on completion of the well is provided with a cementing sub that is located a short distance above the mud line. The cementing sub includes a protective sleeve around a joint in the casing closed at each of its ends against the outer surface of the casing to isolate that joint from fluids in the surrounding annulus. When it is decided to leave the well, all of the strings of casing are out just above the joint within the cementing sub to leave a portion of the casing in the threaded joint. When it is desired to tie back into the well, the portion of the casing in the joint within the cementing sub is removed to leave threads in condition for-receiving a string of casing for extending the well from the mud line to a production platform.

In the drawings:

FIG. 1 is a diagrammatic vertical sectional view of a well incorporating this invention after the well has been drilled to total depth and the desired casing and liners run.

FIG. 2 is a diagrammatic vertical sectional view of the well illustrated in FIG. 1 after cutting off the well a short distance above the mud line.

FIG. 3 is an enlarged vertical sectional view of the cementing sub used in this invention.

FIG. 4 is a diagrammatic vertical sectional view of a well after a production platform has been set above the well and the well has been completed.

Referring to FIG. 1, the platform of a mobile drilling rig is illustrated positioned above an offshore well. The drilling rig and associated equipment on the drilling rig have not been shown since they are not part of this invention. The drilling rig can be of any type, such as a jack-up rig or a floating rig, suitable for drilling in water 100 feet or more deep. This invention also can be used in shallower wells, but is more advantageous in wells that require expensive drilling rigs. The well indicated generally by reference numeral 12 has conductor pipe 14 driven to a suitable depth below the mud line 16. A common practice is for the conductor pipe to be driven to refusal.

Extending down through conductor pipe 14 is a string of surface casing 18. The hole for the surface casing is drilled to a depth determined by the usual conditions for a particular well such as to protect fresh water sands from contamination and confine any shallow high pressure zones that may be encountered. Surface casing I8 is then cemented in place. Cementing can be accomplished by the conventional method of displacing cement down through the casing and up through the annulus between the surface casing and the conductor pipe to the surface, or by grouting into the annulus through a small line.

After the surface casing 18 has been set, the well is drilled to a depth again determined by conditions existing in the particular well, and protector casing 20 set in the well. The protector casing 20 is cemented with cement returns to the surface. Included in the protector casing 20 is a cementing sub 22. The sub is located in the string of protector casing to position the sub a short distance above the mud line 16 when the casing is set in place.

The next smaller string of casing in the well 12 illustrated in the drawings is oil casing 24. The oil casing may extend to the total depth of the well or may extend only part of the way to the bottom of the well as shown in FIG. 1. In the well illustrated in the drawings, the oil casing 24 is cemented to the surface. A liner 26 is suspended within the oil casing 24 by a hanger 28. The annulus between the liner 26 and oil casing 24 above the hanger 28 is filled with a cement plug 30 by grouting for a suitable distance such as feet above the hanger 28. A cement plug 32 is set within the liner above a bridge plug 34. Plugs 30 and 32 isolate the upper end of the well from any high pressure formations that may exist below the lower end of the oil casing 24.

Oil casing 24 includes a cementing sub 36, similar to cementing sub 22, which is positioned at substantially the same distance as cementing plug 22 above the mud line 16. All of the strings of casing extend upwardly above the water line 38 to the platform, and each of the annuli between adjacent strings of casing is filled with cement. In a typical well in which this invention can be used, the conductor pipe 14 has a diameter in the range of 30 to 36 inches and extends to a depth of feet below the mud line. Surface casing 18 has a diameter of 20 inches and extends to a depth of 400 feet below the mud line. The protector casing 20 has a diameter of 13-% inches and extends to a depth of 3,000 feet below the mud line. The oil casing 24 has a diameter of 9-% inches and extends to a depth of 7,000 feet below the mud line, and the 7-inch liner has a diameter of 7 inches and extends to the total depth of the well. The casing sizes and depths set forth in the preceding sentences are merely examples of a typical well in which this invention is useful. This invention is in no way limited to use with those casing sizes or depths.

Referring to FIG. 3, the cementing sub 36 consists of a sleeve 40 having a diameter larger than the outer diameter of the oil casing 24 and smaller than the inner diameter of the protector casing 20 to provide space through which cement can flow into the annulus 50 above the cementing sub 36. The sleeve 40 is closed at its lower end by a bottom plate 44 welded to the outer surface of the oil casing 24 and at its upper end by top plate 46 also welded to the outer surface of casing 44. Within the cementing sub 36 is a threaded joint 48 of cas ing string 24 with the box end of the joint opening upwardly to receive the downwardly extending pin end of the joint of casing 24 that extends upwardly above the cementing sub. Cementing sub 36 isolates joint 48 from fluids in annulus 42 between the protector casing and the oil casing. Cementing sub 22 is similar to cementing sub 36, but isolates the joint within sub 22 from fluids in the annulus 52 between the protector casing and surface casing.

When a decision is made to leave the well and to return later for completing the well, a buoy cable clamp with pad eyes 53 is installed around the conductor pipe 14 below the level at which the casing is to be cut. A multistring casing cutter is run inside the oil casing 24 to a level just above the joint 48, and the casing strings 24, 20, 18 and 14 cut at a level through the cementing subs 22 and 36 to leave a section of casing 24 extending upwardly from joint 48 and a corresponding section of casing extending upwardly from the joint in cementing sub 36. That section of casing is indicated in FIG. 2 by reference numeral 54 and is hereinafter referred to as a dutchman. Similarly, a dutchman originally part of easing string is left in the cementing sub 22. A weighted cap 56 is then lowered onto the well and a buoy 60 is connected by line 62 to pad eye clamps 53 to mark the location of the well.

When it is desired to return to the well, a production platform 61 illustrated in FIG. 4 is set above the well and the cap 56 removed from the well by connecting a line to eye 58. A spear is run from a workover rig into oil casing 24 to engage dutchman 54 and remove the dutchman to expose threads 48. Thereafter, a suitable tool such as an overshot is run into the well to engage the dutchman in cementing sub 22 and remove it from the upwardly opening threaded joint within cementing sub 22. Ordinarily, the thin layer of cement between sleeve 40 and the dutchman in cementing sub 22 is easily broken, but if necessary a core-type drill can be used to drill out that cement to allow removal of the dutchman. Protector casing is then lowered from the platform 61 and connected into the upwardly opening threaded end of casing 20. A suitable casing head 62 is installed on the upper end of casing 22. Thereafter, casing is run through the casing head 62 and connected to joint 48 to provide an extension of oil casing 24 extending upwardly to the casing head. Blowout preventers, not shown, are installed, and the plug 32 and bridge plug 34 are drilled from riser 26 which places the well in condition for normal completion.

in the description of this invention, cementing subs have been used on the protector casing and the oil casing. In some instances, for instance in wells in shallow water, it may not be necessary to fill the annulus between casing 24 and casing 20 all of the way to the surface. In that event, a joint can be included in the oil casing string below the level ofcementing sub 22 that permits the casing above that joint to be removed when the decision is made to leave the well, Then when it is desired to tie into the well, casing can be screwed directly into the open threaded end of the oil casing 24. It may also be desirable in some instances to run the surface casing 18 up to the platform 61 when the well is completed. In that event, a cementing sub can be installed in casing 18 in addition to the protector casing 20 and oil casing 24.

With the method and apparatus of this invention, the annulus between the successive strings of casing from the mud line to the platform of the drilling rig is maintained full of cement during the entire drilling of the borehole. Moreover, there is no manipulation of casing which might result in leakage from the small diameter casing to larger diameter casing. The cementing subs installed in the casing strings to permit easy tiein to the wells are simple and can be manufactured at a very low cost compared to the equipment heretofore available.

I claim:

1. A method for drilling an offshore well, cutting it off a short distance above the mud line, and leaving it in condition for tie-back comprising drilling the borehole of a well to a desired depth, setting a first string of casing in the borehole, drilling the borehole of the well below the lower end of the first string of easing, running a second string of casing into the borehole below the lower end of the first string of casing, shielding ajoint located a short distance above the mud line in the second string of casing, filling the annulus surrounding the second string of casing with cement all of the way to the upper end of the first string of casing, suspending the second string of casing until the cement sets, cutting the casing through the shielding and above the shielded joint to leave a dutchman in the shielded joint.

2. A method as set forth in claim 1 in which plugs are set in the second string of casing to isolate the upper end of the second string of casing at the level of the mud line from productive formations.

3. A method of cutting off an offshore well slightly above the mud line to leave it in condition for tying back comprising setting a first string of casing in the well to a desired depth, suspending a second string of easing of smaller diameter than the first string of casing within the first string of casing, said second string of casing extending below the lower end of the first string of casing, isolating a joint in the second string of casing a short distance above the mud line from the annulus between the first string of casing and second string of casing, filling with cement the annulus between the first string of casing and the second string of casing to the upper end of the first string of easing, setting a plug within the second string of casing to isolate the second string of casing at the level of the mud line from productive formations, cutting the second string of easing through the isolated portion to leave a dutchman in the isolated joint, cutting the first string of casing at substantially the level at which the second string of casing was cut, and removing the first and second strings of easing above the cutting.

4. A method of cutting off an offshore well a short distance above the mud line to leave the well in condition for tie-back comprising setting a first string of casing in the well, running a second string of casing downwardly through the first string of casing to a level below the lower end of the first string of casing, isolating a portion of the second string of casing a short distance above the mud line from the annulus between the first string of casing and the second string of casing, said isolated portion of the second string of casing including a joint, filling the annulus between the first string of casing and second string of casing with cement to the upper end of the first string of casing, running a third string of casing into the well to a level below the lower end of the second string of casing, filling the annulus between the second and third strings of casing with cement to a level below the isolated portion ofthe second string of easing, setting a plug in the third string of casing, disconnecting the third string of easing above the level of the cement in the annulus between the second string of casing and the third string of casing, removing the disconnected upper portion of the third string of casing from the well, cutting the first string of casing and second string of casing at a level through the shielded portion of the second string of casing and above the joint therein to leave a dutchman in the second string of casing, and removing from the well the portions of the first string of casing and second string of easing above the cutting.

5. An offshore well structure adapted for cutting off above the mud line and re-entering comprising a plurality of concentric strings of casing extending downwardly into the well, cement filling the annuli between the concentric strings of casing, a cementing sub in at least the smallest string of easing extending to the level at which the sub is cut off, said sub comprising a sleeve surrounding said smallest casing, closure members at each end of the sleeve connected to the outer surface ofsaid smallest casing, and a casingjoint within the sleeve having connected therein a section of easing extending upwardly beyond the sleeve, said sleeve having an outer diameter smaller than the inner diameter of the next larger string of casing to permit flow through the annulus between said smallest casing and the next larger string of casing.

6. An offshore well structure adapted for cutting off a short distance above the mud line and subsequent re-entry comprising conductor pipe extending downwardly into the well, surface casing extending downwardly through the conductor pipe to a level below the lower end of the conductor pipe, protector casing extending downwardly through the surface casing to a level below the surface casing, oil casing extending downwardly through the protector casing to a level below the end of the protector casing, cement filling the annuli between each of the successive strings of easing, a cementing sub in the oil casing a short distance above the mud line, said cementing sub comprising a sleeve surrounding the oil casing, closure members at each end of the sleeve connected to the oil casing, and a threaded joint within the sleeve having a section of the oil casing screwed therein.

7. Apparatus as set forth in claim 6 in which a second cementing sub is in the protector casing at substantially the same level as the cementing sub in the oil casing.

8. A cementing sub for installation in a string casing in a well comprising an upper section of casing and a lower section of easing, said upper and lower sections of easing being of the same internal and external diameter as the casing in the string, a joint joining the upper section of casing and the lower section of casing comprising threads on the upper end of the lower section of casing engaging threads on the lower end of the upper section of easing, the lower section of casing adapted at its lower end for connection below the sub to the string of casing and the upper section of easing adapted at its upper end for connection to the string of easing above the sub. a sleeve surrounding and spaced from the upper and lower sections and extending longitudinally above and below the joint, and closure members joining the upper end of the sleeve and the outer surface of the upper section of casing and the lower end of the sleeve and the outer surface of the lower section of casing to prevent flow from outside of the sub into the joint. 

1. A method for drilling an offshore well, cutting it off a short distance above the mud line, and leaving it in condition for tie-back comprising drilling the borehole of a well to a desired depth, setting a first string of casing in the borehole, drilling the borehole of the well below the lower end of the first string of casing, running a second string of casing into the borehole below the lower end of the first string of casing, shielding a joint located a short distance above the mud line in the second string of casing, filling the annulus surrounding the second string of casing with cement all of the way to the upper end of the first string of casing, suspending the second string of casing until the cement sets, cutting the casing through the shielding and above the shielded joint to leave a dutchman in the shielded joint.
 2. A method as set forth in claim 1 in which plugs are set in the second string of casing to isolate the upper end of the second string of casing at the level of the mud line from productive formations.
 3. A method of cutting off an offshore well slightly above the mud line to leave it in condition for tying back comprising setting a first string of casing in the well to a desired depth, suspending a second string of casing of smaller diameter than the first string of casing within the first string of casing, said second string of casing extending below the lower end of the first string of casing, isolating a joint in the second string of casing a short distance above the mud line from the annulus between the first string of casing and second string of casing, filling with cement the annulus between the first string of casing and the second string of casing to the upper end of the first string of casing, setting a plug within the second string of casing to isolate the second string of casing at the level of the mud line from productive formations, cutting the second string of casing through the isolated portion to leave a dutchman in the isolated joint, cutting the first string of casing at substantially the level at which the second string of casing was cut, and removing the first and second strings of casing above the cutting.
 4. A method of cutting off an offshore well a short distance above the mud line to leave the well in condition for tie-back comprising setting a first string of casing in the well, running a second string of casing downwardly through the first string of casing to a level below the lower end of the first string of casing, isolating a portion of the second string of casing a short distance above the mud line from the annulus between the first strIng of casing and the second string of casing, said isolated portion of the second string of casing including a joint, filling the annulus between the first string of casing and second string of casing with cement to the upper end of the first string of casing, running a third string of casing into the well to a level below the lower end of the second string of casing, filling the annulus between the second and third strings of casing with cement to a level below the isolated portion of the second string of casing, setting a plug in the third string of casing, disconnecting the third string of casing above the level of the cement in the annulus between the second string of casing and the third string of casing, removing the disconnected upper portion of the third string of casing from the well, cutting the first string of casing and second string of casing at a level through the shielded portion of the second string of casing and above the joint therein to leave a dutchman in the second string of casing, and removing from the well the portions of the first string of casing and second string of casing above the cutting.
 5. An offshore well structure adapted for cutting off above the mud line and re-entering comprising a plurality of concentric strings of casing extending downwardly into the well, cement filling the annuli between the concentric strings of casing, a cementing sub in at least the smallest string of casing extending to the level at which the sub is cut off, said sub comprising a sleeve surrounding said smallest casing, closure members at each end of the sleeve connected to the outer surface of said smallest casing, and a casing joint within the sleeve having connected therein a section of casing extending upwardly beyond the sleeve, said sleeve having an outer diameter smaller than the inner diameter of the next larger string of casing to permit flow through the annulus between said smallest casing and the next larger string of casing.
 6. An offshore well structure adapted for cutting off a short distance above the mud line and subsequent re-entry comprising conductor pipe extending downwardly into the well, surface casing extending downwardly through the conductor pipe to a level below the lower end of the conductor pipe, protector casing extending downwardly through the surface casing to a level below the surface casing, oil casing extending downwardly through the protector casing to a level below the end of the protector casing, cement filling the annuli between each of the successive strings of casing, a cementing sub in the oil casing a short distance above the mud line, said cementing sub comprising a sleeve surrounding the oil casing, closure members at each end of the sleeve connected to the oil casing, and a threaded joint within the sleeve having a section of the oil casing screwed therein.
 7. Apparatus as set forth in claim 6 in which a second cementing sub is in the protector casing at substantially the same level as the cementing sub in the oil casing.
 8. A cementing sub for installation in a string casing in a well comprising an upper section of casing and a lower section of casing, said upper and lower sections of casing being of the same internal and external diameter as the casing in the string, a joint joining the upper section of casing and the lower section of casing comprising threads on the upper end of the lower section of casing engaging threads on the lower end of the upper section of casing, the lower section of casing adapted at its lower end for connection below the sub to the string of casing and the upper section of casing adapted at its upper end for connection to the string of casing above the sub, a sleeve surrounding and spaced from the upper and lower sections and extending longitudinally above and below the joint, and closure members joining the upper end of the sleeve and the outer surface of the upper section of casing and the lower end of the sleeve and the outer surface of the lower section of casing to Prevent flow from outside of the sub into the joint. 